Additive for increasing the density of a fluid for casing annulus pressure

ABSTRACT

A method of controlling the pressure of a casing annulus in a subterranean well that includes injecting into the casing annulus a composition including a base fluid and a polymer coated colloidal solid material. The polymer coated colloidal solid material includes: a solid particle having an weight average particle diameter (d 50 ) of less than two microns, and a polymeric dispersing agent coated onto the surface of the solid particle during the cominution (i.e. grinding) process utilized to make the colloidal particles. The polymeric dispersing agent may be a water soluble polymer having a molecular weight of at least 2000 Daltons. The solid particulate material may be selected from materials having of specific gravity of at least 2.68 and preferably the solid particulate material may be selected from barium sulfate (barite), calcium carbonate, dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate, combinations and mixtures of these and other similar solids that should be apparent to one of skill in the art.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 10/274,528 filed Oct. 18, 2002, now abandoned.Application Ser. No. 10/274,528 is a continuation-in-part of U.S.application Ser. No. 09/230,302, filed Sep. 10, 1999, now U.S. Pat. No.6,586,372, which is the U.S. national phase application under 35 U.S.C.§.371 of a PCT International Application No. PCT/EP97/003,802, filedJul. 16, 1997 which in turn claims priority under the Paris Conventionto United Kingdom Patent Application No. 9615549.4 filed Jul. 24, 1996.Said applications are expressly incorporated herein by reference intheir entireties.

BACKGROUND OF THE INVENTION

In the offshore oil and gas production industry, there has been a longand unmet need for dealing with a problem known as sustained casingannulus pressure. Sustained casing annulus pressure can be defined asany recorded pressure on casing strings, other than drive or structuralstrings, that cannot be bled to zero. Causes of sustained casing annuluspressure include leaks in tubing, casing, packers, wellhead packoffs,and poor or failed primary cement jobs.

Controlling casing annulus pressure is a significant problem, especiallyin the offshore drilling environment. In those areas of the Gulf ofMexico which are federally regulated, the Minerals Management Serviceguidelines mandate zero pressure above the sea floor at all times, butdo allow for certain types of non-compliant approval to maintainproduction or delay early abandonment. It has been reported that morethan 8000 wells and 11,000 casing strings have been identified withsustained casing annulus pressure in the Gulf of Mexico alone. Of thesereported cases, approximately 30% of these wells require specialdeparture waivers issued by the Minerals Management Service to maintainproduction and all require continuous investment in either remediationor monitoring. Further in recent years, enforcement has become morerestrictive and several operators have been forced to spend millions ofdollars to solve this problem.

Sustained casing annulus pressure can also be a significant safety issuefor oil and gas producing wells. In a recent report, approximately 150Alaskan North Slope wells subject to casing annulus pressure buildupwere shut-down by the operator out of safety concerns. This shut-down ofconsiderable production capacity (reportedly about 6 percent of totalcrude output) was a safety precaution taken in response to the ruptureand fire at a well caused by casing annulus pressure buildup.

One reported low cost method of controlling sustained casing annuluspressure is inserting a flexible hose into the restricted annuli ofouter casing strings so high density fluids can be effectivelydisplaced. Typically these high density fluids include high densitybrines specially formulated for injection and displacement of theexisting fluids in the casing annulus. This displacement of the existingannulus fluid with a heavier (i.e. higher density) brine provides asimple way for an operator to regain control over sustained casingannulus pressures.

Common difficulties with the above method include inserting the flexibletubing to the desired depth without coiling and effectively displacingthe existing casing annulus fluid with the desired heavy brine. Further,it should be appreciated that dilution of the injected fluid andcorrosion caused by the high brine concentration are significantconcerns. Furthermore, high density brines are expensive and poseadditional health, safety and product handling concerns. Further it isknown that heavy brines can cause a non-salt containing water basedpacker fluid to flocculate. This flocculation is reported to not allowthe heavy brine to settle to the bottom of the casing string were it isdesired. Replacement of the heavy brine solution with high densityfluids of suspended solids (such a barite) is generally consideredimpractical because suspending the solids requires fluids of highviscosity which are not easily injected. Small diameter aperturespresent in the valves and other flow and pressure control equipment usedto place casing annular fluids prevent the use of conventional weightingagents because these materials block and plug the narrow restrictions.Despite the continued efforts in this area, there remains and exists anunmet need for fluids that exhibit a high density and do not exhibit theproblems of solids settling or corrosion concerns.

SUMMARY OF THE INVENTION

The present invention is generally directed to fluids useful incontrolling casing annulus pressure, as well as methods for making andmethods of using such fluids. The fluids of the present inventioninclude a polymer coated colloidal solid material that has been coatedwith a polymer added during the comminution (i.e. grinding) process forpreparing the polymer coated colloidal solid material.

One illustrative embodiment of the present invention includes a methodof controlling the pressure of a casing annulus in a subterranean well.In such an illustrative method, the method includes, injecting into thecasing annulus a composition including a base fluid, and a polymercoated colloidal solid material. The polymer coated colloidal solidmaterial includes: a solid particle having an weight average particlediameter (d50) of less than two microns, and a polymeric dispersingagent absorbed to the surface of the solid particle. The polymericdispersing agent is absorbed to the surface of the solid particle duringthe comminution (i.e. grinding) process utilized to make the polymercoated colloidal solid material. The base fluid utilized in the aboveillustrative embodiment can be an aqueous fluid or an oleaginous fluidand preferably is selected from: water, brine, diesel oil, mineral oil,white oil, n-alkanes, synthetic oils, saturated and unsaturatedpoly(alpha- olefins), esters of fatty acid carboxylic acids andcombinations and mixtures of these and similar fluids that should beapparent to one of skill in the art. Suitable and illustrative colloidalsolids are selected such that the solid particles are composed of amaterial of specific gravity of at least 2.68 and preferably areselected from barium sulfate (barite), calcium carbonate, dolomite,ilmenite, hematite, olivine, siderite, strontium sulfate, combinationsand mixtures of these and other suitable materials that should be wellknown to one of skill in the art. In one preferred and illustrativeembodiment, the polymer coated colloidal solid material has a weightaverage particle diameter (d5o) less than 2.0 microns. Another preferredand illustrative embodiment is such that at least 50% of the solidparticles have a diameter less than 2 microns and more preferably atleast 80% of the solid particles have a diameter less than 2 microns.Alternatively, the particle diameter distribution in one illustrativeembodiment is such that greater than 25% of the solid particles have adiameter of less than 2 microns and more preferably greater than 50% ofthe solid particle have a diameter of less than 2 microns. The polymericdispersing agent utilized in one illustrative and preferred embodimentis a polymer of molecular weight of at least 2,000 Daltons. In anothermore preferred and illustrative embodiment, the polymeric dispersingagent is a water soluble polymer is a homopolymer or copolymer ofmonomers selected from the group comprising: acrylic acid, itaconicacid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonicacid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonicacid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate,and wherein the acid monomers may also be neutralized to a salt.

The present invention is also directed to a composition that includes abase fluid and a polymer coated colloidal solid material. The polymercoated colloidal solid material is formulated so as to include a solidparticle having an weight average particle diameter (d₅₀) of less thantwo microns; and a polymeric dispersing agent absorbed to the surface ofthe colloidal solid particle.

In addition to the above, the present invention is directed to a methodof making the polymer coated colloidal solid materials utilized anddescribed herein. Such an illustrative method includes grinding a solidparticulate material and a polymeric dispersing agent for a sufficienttime to achieve an weight average particle diameter (d₅₀) of less thantwo microns; and so that the polymeric dispersing agent is absorbed tothe surface of the solid particle. Preferably the illustrative grindingprocess is carried out in the presence of a base fluid that is either anaqueous fluid or an oleaginous fluid.

These and other features of the present invention are more fully setforth in the following description of preferred or illustrativeembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The description is presented with reference to the accompanying drawingwhich is a graphical representation of the particle diameterdistribution of the colloidal barite of the present invention comparedto that of API barite.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

One of the most important functions of a fluid of the present inventionis to contribute to the stability of the well bore, and control the flowof gas, oil or water from the pores of the formation in order toprevent, for example, the flow or blow out of formation fluids or thecollapse of pressured earth formations. The column of fluid in the holeexerts a hydrostatic pressure proportional to the depth of the hole andthe density of the fluid. High pressure formations may require a fluidwith a specific gravity of up to 3.0.

A variety of materials are presently used to increase the density offluids in the oil and gas well drilling and production industry. Suchmaterials include dissolved salts such as sodium chloride, calciumchloride and calcium bromide. Alternatively powdered minerals such asbarite, calcite and hematite are added to a fluid to form a suspensionof increased density. It is also known to utilize finely divided metalsuch as iron as a weight material. In this connection, PCT PatentApplication WO85/05118 discloses a drilling fluid where the weightmaterial includes iron/steel ball-shaped particles having a diameterless than 250 microns and preferentially between 15 and 75 microns. Ithas also been proposed to use calcium or iron carbonate (see for exampleU.S. Pat. No. 4,217,229).

One desirable characteristic of the fluids utilized in the context ofthe present invention is that the particles form a stable suspension,and do not readily settle out. A second desirable characteristic is thatthe suspension should exhibit a low viscosity in order to facilitatepumping and to minimize the generation of high pressures. Anotherdesireable characteristic is that the fluid slurry should exhibit lowfiltration rates (fluid loss).

Conventional weighting agents such as powdered barium sulfate (“barite”)exhibit an average particle diameter (d₅₀) in the range of 10-30microns. It should be well known to one of skill in the art thatproperties of conventional weighting agents, barite in particular, aresubject to strict quality control parameters established by the AmericanPetroleum Institute (API). To suspend these materials adequatelyrequires the addition of a gellant or viscosifier such as bentonite forwater based fluids, or organically modified bentonite for oil basedfluids. Polymeric viscosifiers such as xanthan gum may be also added toslow the rate of the sedimentation of the weighting agent. However, oneof skill in the art should appreciate that as more gellant is added toincrease the suspension stability, the fluid viscosity (plasticviscosity) increases undesirably resulting in reduced pumpability.

The sedimentation (or “sag”) of particulate weighting agents isimportant for maintaining or controlling pressures in a wellbore, awellbore annulus or casing annuli. Should there be a gradual separationof the solid and liquid phases of a fluid over a period of time, thedensity of the fluid in the wellbore, the annulus or casing annulusbecomes inhomogeneous and the hydrostatic pressure exerted on thewellbore formations may be less than the pressure of wellbore formationfluids, resulting in well control issues and potentially a blow out.

This is no less important in deep high pressure wells where high densityfluids may be required to control the casing annulus pressure. Again,the stability of the suspension is important in order to maintain thehydrostatic head to avoid a blow out. One of skill in the art shouldunderstand and appreciate that the two objectives of having a lowviscosity fluid that is readily pumped into the casing annulus plusminimal sag of any weighting material present can be difficult toreconcile.

It is known that reduced particle sedimentation rates can be obtained byreducing the particle size used. However, the conventional view in thedrilling industry is that reducing the particle size causes anundesirable increase in viscosity. The increase in viscosity is reportedin the literature as being caused by an increase in the surface area ofthe particles causing increased adsorption of water and thus athickening of the suspension. For example, “Drilling and DrillingFluids” Chilingarian G. V. and Vorabutor P. 1981, pages 441-444 states:“The difference in results (i.e. increase in , plastic viscosity) whenparticle size is varied in a mud sluny is primarily due to magnitude ofthe surface area, which determines the degree of adsorption (tying up)of water. More water is adsorbed with increasing area.” The main thrustof the teachings is that colloidal fines due to their nature of having ahigh surface area to volume ratio will absorb significantly more waterand so decrease the fluidity of the mud. The same argument or concept ispresented in “Drilling Practices Manual” edited by Moore, pages 185-189(1986). Walter F. Rogers in “Composition and Properties of Oil WellDrilling Fluids” in pages 148-151 (1953), presents the same argumentwhere the higher the number of barite particles per gram (hence particlesize), the higher and more detrimental the viscosity. Malachosky inPetroleum Engineer International, July 1986 pages 40-43 discusses thedetrimental influence on fluid properties of colloidal barite, and hightreatment costs because of the high surface area. This understandingthat small particle size is detrimental is well known in the prior artand is reflected and illustrated by the API specification for barite asa drilling fluid additive which limits the particle content % w/w below6 microns to 30% maximum in order to minimize viscosity increases.Further as is illustrated on page 190 of “Drilling Practices Manual”edited by Moore, which has a bar graph showing that the percent byweight of particle below 2 microns (i.e. colloidal solids) for APIbarite is less than 15% in all cases shown.

It is therefore very surprising that the products of this invention,which comprise particles very finely ground to an average particlediameter (d₅₀) of less than two microns, provide fluids of reducedplastic viscosity in combination with greatly reducing sedimentation orsag.

The additives of this invention comprise dispersed solid colloidalparticles with a weight average particle diameter (d₅₀) of less than 2microns that are coated with a polymeric defloculating agent ordispersing agent. The fine particle size will generate suspensions orslurries that will show a reduced tendency to sediment or sag, whilstthe polymeric dispersing agent on the surface of the particle controlthe inter-particle interactions and thus will produce lower rheologicalprofiles. It is the combination of fine particle size and control ofcolloidal interactions that reconciles the two objectives of lowerviscosity and minimal sag.

According to the present invention, the polymeric dispersant is coatedonto the surface of the particulate weighting agent during the grindingprocess utilized to form the colloidal particle. It is believed thatduring the course of the grinding process, newly exposed particlesurfaces become polymer coated thus resulting in the propertiesexhibited by the colloidal solids of the present invention. Experimentaldata has shown that colloidal solid material created in the absence ofthe polymeric dispersant results in a concentrated slurry of smallparticles that forms an unpumpable paste or gel. According to theteachings of the present invention, a polymeric dispersant is addedduring the grinding process. It is believed that this differenceprovides an advantageous improvement in the state of dispersion of theparticles compared to post addition of the polymeric dispersant to fineparticles. According to a preferred embodiment, the polymeric dispersantis chosen so as it provides the suitable colloidal inter-particleinteraction mechanism to make it tolerant to a range of common wellborecontaminants, including salt.

A method of grinding a solid material to obtain the solid colloidalparticles of the present invention is well known for example fromBritish Patent Specification No. 1,472,701 or No. 1,599,632. The mineralin an aqueous suspension is mixed with a polymeric dispersing agent andthen ground within an agitated fluidized bed of a particulate grindingmedium for a time sufficient to provide the required particle sizedistribution. An important preferred embodiment aspect of the presentinvention is the presence of the dispersing agent in the step of “wet”grinding the mineral. This prevents new crystal surfaces formed duringthe grinding step from forming agglomerates which are not so readilybroken down if they are subsequently treated with a dispersing agent.

According to a preferred embodiment of the present invention, theweighting agent of the present invention is formed of particles that arecomposed of a material of specific gravity of at least 2.68. Materialsof specific gravity greater than 2.68 from which colloidal solidparticles that embody one aspect of the present invention include one ormore materials selected from but not limited to barium sulfate (barite),calcium carbonate, dolomite, ilmenite, hematite or other iron ores,olivine, siderite, strontium sulfate. Normally the lowest wellbore fluidviscosity at any particular density is obtained by using the highestdensity colloidal particles. However other considerations may influencethe choice of product such as cost, local availability and the powerrequired for grinding.

A preferred embodiment of this invention is for the weight averageparticle diameter d₅₀ of the colloidal solid particles to be less than2.0 microns wherein at least 50% of the solid particles have a diameterless than 2 microns. More preferably at least 80% of the colloidal solidparticles have a diameter less than 2 microns. Alternatively, theparticle diameter distribution in one illustrative embodiment is suchthat greater than 25% of the colloidal solid particles have a diameterof less than 2 microns and more preferably greater than 50% of thecolloidal solid particles have a diameter of less than 2 microns. Thiswill enhance the suspension's characteristics in terms of sedimentationor sag stability without the viscosity of the fluid increasing so as tomake it unpumpable.

The polymer coated colloidal particles according the invention may beprovided as a concentrated slurry either in an aqueous medium or anoleaginous liquid. In the latter case, the oleaginous liquid should havea kinematic viscosity of less than 10 centistokes (10 mm²/s) at 40° C.and, for safety reasons, a flash point of greater than 60° C. Suitableoleaginous liquids are for example diesel oil, mineral or white oils,n-alkanes or synthetic oils such as alpha-olefin oils, ester oils orpoly(alpha-olefins).

Where the polymer coated colloidal particles are provided in an aqueousmedium, the dispersing agent may be, for example, a water-solublepolymer of molecular weight of at least 2,000 Daltons. The polymer is ahomopolymer or copolymer of any monomers selected from (but not limitedto) the class comprising: acrylic acid, itaconic acid, maleic acid oranhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylicphosphate esters, methyl vinyl ether and vinyl acetate. The acidmonomers may also be neutralized to a salt such as the sodium salt.

It has been found that when the dispersing agent is added during thecomminution process (i.e. grinding), intermediate molecular weightpolymers (in the range 10,000 to 200,000 for example) may be usedeffectively. Intermediate molecular weight dispersing agents areadvantageously less sensitive to contaminants such as salt, clays, andtherefore are well adapted to wellbore fluids.

Where the colloidal particles are provided in an oleaginous medium, thedispersing agent may be selected for example among carboxylic acids ofmolecular weight of at least 150 such as oleic acid and polybasic fattyacids, alkylbenzene sulphonic acids, alkane sulphonic acids, linearalpha-olefin sulphonic acid or the alkaline earth metal salts of any ofthe above acids, phospholipids such as lecithin, synthetic polymers suchas Hypermer OM-1 (trademark of ICI).

This invention has a surprising variety of applications in drillingfluids, cement and cementing fluids, spacer fluids, other high densityfluids and coiled tubing drilling fluids as well as the uses of themethods of the present invention in controlling casing annulus pressure.The new particulate weighting agents have the ability to stabilize thelaminar flow regime, and delay the onset of turbulence. It is possibleto formulate fluids for several applications that will be able to bepumped faster before turbulence is encountered, so giving essentiallylower pressure drops at equivalent flow rates. This ability to stabilizethe laminar flow regime although surprising is adequately demonstratedin heavy density muds of 20 pounds per gallon (2.39 g/cm³) or higher.Such high density muds using conventional weighting agents, with aweight average particle diameter of 10 to 30 microns, would exhibitdilatancy with the concomitant increase in the pressure drops due to theturbulence generated. The ability of the new weighting agent tostabilize the flow means that high density fluids with acceptablerheology are feasible with lower pressure drops.

The fluids of the present invention may also be used in non-oilfieldapplications such as dense media separating fluid (to recover ore forexample) or as a ship's ballast fluid.

The following examples are to illustrate the properties and performanceof the wellbore fluids of the present invention though the invention isnot limited to the specific embodiments showing these examples. Alltesting was conducted as per API RP 13 B where applicable. Mixing wasperformed on Silverson L2R or Hamilton Beach Mixers. The viscosity atvarious shear rates (RPM's) and other rheological properties wereobtained using a Fann viscometer. Mud weights were checked using astandard mud scale or an analytical balance. Fluid loss was measuredwith a standard API fluid loss cell

In expressing a metric equivalent, the following U.S. to metricconversion factors are used: 1 gal=3.785 litres; 1 lb.=0.454 kg; 1lb./gal (ppg)=0.1198 g/cm³ 1 bbl=42 gal; 1 lb./bbl (ppb)=2.835 kg/m³; 1lb/100 ft.sup.2=0,4788 Pa.

These tests have been carried out with different grades of groundbarite: a standard grade of API barite, having a weight average particlediameter (D₅₀) of about 20 microns; an untreated barite (M) having anaverage size of 3-5 microns made by milling/grinding barite while in thedry state and in the absence of a dispersant; and colloidal bariteaccording the present invention (with a D₅₀ from 0.5 microns to 2.0microns), with a polymeric dispersant included during a “wet” grindingprocess.

The corresponding particle size distributions are shown FIG. 1. As shownin FIG. 1, one of skill in the should understand and appreciate that thecolloidal barite of the present invention has a particle sizedistribution that is very different from that of API barite.Specifically one should be able to determine that greater than about 90%(by volume) of the colloidal barite of the present invention has aparticle diameter less than about 5 microns. In contrast, less than 15percent by volume of the particles in API specification barite have aparticle diameter less than 5 microns.

The polymeric dispersant is IDSPERSE.TM. XT an anionic acrylicter-polymer of molecular weight in the range 40,000-120,000 withcarboxylate and other functional groups commercially available from M-ILLC. Houston, Tex. This preferred polymer is advantageously stable attemperatures up to 200° C., tolerant to a broad range of contaminants,provides good filtration properties and does not readily desorb off theparticle surface.

EXAMPLE 1

22 ppg [2.63 g/cm³] fluids based on barium sulfate and water wereprepared using standard barite and colloidal barite according to theinvention. The 22 ppg slurry of API grade barite and water was made withno gelling agent to control the inter-particle interactions (Fluid #1).Fluid #2 is also based on standard API barite but with a post-additionof two pounds per barrel (5.7 kilograms per cubic meter) IDSPERSE XT.Fluid #3 is 100% new weighting agent with 67% w/w of particles below 1micron in size and at least 90% less than 2 microns. The results areprovided in table 1.

TABLE I Viscosity at various shear rates (rpm of agitation): Yield Dialreading or “Fann Units” for: Plastic Point 600 300 200 Viscosity lb/100ft² # rpm rpm rpm 100 rpm 6 rpm 3 rpm mPa · s (Pascals) 1 250 160 124 9225 16 90 70 (34) 2 265 105 64 26 1 1 160 −55 (−26) 3 65 38 27 17 3 2 2711 (5) 

For Fluid #1 the viscosity is very high and the slurry was observed tofilter very rapidly. (If further materials were added to reduce thefluid loss, the viscosity would have increased yet further). This systemsags significantly over one hour giving substantial free water (ca. 10%of original volume).

Post addition of two pounds per barrel [5.7 kg/cm³] of IDSPERSE XT toconventional API barite (Fluid #2) reduces the low shear rate viscosityby controlling the inter-particle interactions. However due to theparticle concentration and average particle size the fluid exhibitsdilatency, which is indicated by the high plastic viscosity and negativeyield point. This has considerable consequences on the pressure dropsfor these fluids while pumping. That is to say the ability to pump thisfluid is substantially reduced due to the high viscosity. The fluid #2sags immediately on standing.

By contrast, Fluid #3 exhibits an excellent, low, plastic viscosity. Thepresence of the dispersing polymer controls the inter-particleinteractions, so making fluid #3 pumpable and not a gel. Also the muchlower average particle size has stabilized the flow regime and is nowlaminar at 1000s⁻¹ demonstrated by the low plastic viscosity andpositive yield point.

EXAMPLE 2

Experiments were conducted to examine the effect of the post addition ofthe chosen polymer dispersant to a slurry comprising weighting agents ofthe same colloidal particle size. A milled barite (D₅₀˜4 μm) and amilled calcium carbonate (70% by weight of the particles of less than 2μm) were selected, both of which are of similar particle size to theinvention related herein. The slurries were prepared at an equivalentparticle volume fraction of 0.282 and compared to the product of thepresent invention (new barite). See table II.

The rheologies were measured at 120° F. (49° C.), thereafter an additionof 6 ppb (17.2 kg/m³) IDSPERSE XT was made. The rheologies of thesubsequent slurries were finally measured at 120° F. (see table III)with additional API fluid loss test.

TABLE II Volume # Material Dispersant Density (ppg) Fraction wt/wt 4 Newbarite while grinding 16.0 [1.92 g/cm³] 0.282 0.625 5 Milled barite none16.0 [1.92 g/cm³] 0.282 0.625 6 Milled barite post-addition 16.0 [1.92g/cm³] 0.282 0.625 7 Calcium none 12.4 [1.48 g/cm³] 0.282 0.518Carbonate 8 Calcium post-addition 12.4 [1.48 g/cm³] 0.282 0.518Carbonate

TABLE III Viscosity at various shear rates (rpm of agitation): PlasticYield Dial reading or “Fann Units” for: Viscos- Point API 600 300 200100 6 ity lb/ Fluid # rpm rpm rpm rpm rpm 3 rpm mPa · s 100 ft² Loss 412 6 4 2 6 0 11 5 os os os os os os 6 12 6 4 2 6 0 total¹ 7 os os 260221 88 78 8 12 6 4 3  1  1 6 0 total² ¹total fluid loss in 26 minutes;²total fluid loss in 20 minutes

No filtration control is gained from post addition of the polymer asrevealed by the total fluid loss in the API test.

One of skill in the art should appreciate and know that the performanceparameters of major importance are: low rheology, including plasticviscosity (PV), yield point (YP), gel strengths; minimal rheologyvariation between initial and heat aged properties; minimal fluid lossand minimal sag or settlement. Sag is quantified in the followingexamples by separately measuring the density of the top half and bottomhalf of an aged fluid sample, and a dimensionless factor calculatedusing the following equation:Sag Factor=(density of the top half)/(density of the top half+density ofthe bottom half)

A factor of 0.50 indicates zero solids separation and a no densityvariation throughout the fluid sample. A sag factor greater than 0.52 isnormally considered unacceptable solids separation.

EXAMPLE 3

In the following example, two 13.0 ppg fluid formulations are compared,one weighted with conventional API barite and the second weighted withpolymer coated colloidal barite (PCC barite) made in accordance with theteachings of the present invention, as a 2.2 sg liquid slurry. Otheradditives in the formulation are included to provide additional controlof pH, fluid loss, rheology, inhibition to reactive shale andclaystones. These additives are available from M-I Drilling Fluids.

PRODUCT Fluid A Fluid B PCC barite lbs/bbl 320.0 API barite lbs/bbl238.1 Freshwater lbs/bbl 175.0 264.2 Soda Ash lbs/bbl 0.4 0.4 Celpol ESLlbs/bbl 3.5 4.2 Flotrol lbs/bbl 3.5 0 Defoam NS lbs/bbl 0.4 0 KCllbs/bbl 32.9 36.1 Glydril; MC lbs/bbl 10.5 10.5 Duotec NS lbs/bbl 0.11.4

The fluids were heat aged statically for 48 hrs at 104° F. with thefollowing exemplary results.

FANN 35 Fluid A Fluid B Reading (120° F.) Initial Aged Initial Aged 600rpm 56 62 73 65 300 rpm 36 41 52 47 200 rpm 28 33 42 39 100 rpm 19 23 3129  6 rpm 5 7 11 10  3 rpm 4 6 9 8 PV (cps) 20 21 21 18 YP (lbs/100 sq.ft) 16 20 31 29 10 sec gel (lbs/100 sq. ft) 5 7 10 9 10 min gel (lbs/100sq. ft) 8 8 12 Sag Factor 0.50 0.58

One of skill in the art should appreciate upon review of the aboveresults that Fluid A, formulated with the polymer coated colloidalbarite, had no solids separation with a sag factor of zero with arheological profile much lower than a fluid weighted with conventionalAPI barite.

EXAMPLE 4

In the following example, a 14.0 ppg Freshwater fluid was chosen tocompare the properties of fluids formulated with a polymer coatedcolloidal barite; an uncoated colloidal barite and a conventional APIbarite. Fluid A was formulated with the polymer coated colloidal bariteof this invention. Fluid B was formulated with conventional API barite.Fluid C was formulated with a commercial grade of non coated colloidalbarite, of median particle size of 1.6 microns available from HighwoodResources Ltd., Canada. Post grinding addition of the coating polymer ofthe invention are included in the formulation of Fluids B and C tomaintain the fluid in a deflocculated condition.

PRODUCT Fluid A Fluid B Fluid C PCC barite lbs/bbl 407 API baritelbs/bbl 300 Sparwite W-5HB lbs/bbl 310 Freshwater lbs/bbl 182 276 274Idsperse XT 6.0 6.2 XCD Polymer lbs/bbl 0.5 0.6 0.5 DUAL-FLO lbs/bbl 7 57 Bentonite lbs/bbl 10 10 10

Samples of fluids A, B and C were purposely contaminated with bentoniteto simulate the inclusion of naturally drilled solids in theformulation. The samples were heat aged dynamically at 150.degree. F for16 hrs. Exemplary and representative results after aging are shownbelow.

Fluid A Fluid B Fluid C FANN 35 No With No With No With Reading (100°F.) Bentonite Bentonite Bentonite Bentonite Bentonite Bentonite 600 rpm74 76 78 205 94 off scale 300 rpm 48 49 51 129 58 off scale 200 rpm 3839 39 100 45 100 rpm 27 27 27 67 29  6 rpm 8 8 8 20 7  3 rpm 6 6 6 19 6PV (cps) 26 27 27 76 36 YP (lbs/100 sq. ft) 22 22 24 53 22 10 sec gel 76 6 17 6 (lbs/100 sq. ft) 10 min gel 9 9 7 20 7 (lbs/100 sq. ft) APIFluid Loss 3.5 3.0 4 3.9 (ml/30 min)

Upon review of the above data, one of skill in the art should appreciatethat the properties of Fluid A remain essentially unchanged, while theFluid B became very viscous, whereas, the rheology of Fluid C formulatedwith non coated colloidal barite after aging was too viscous to measure.

EXAMPLE 5

A further comparison between a polymer coated colloidal barite of thisinvention and conventional API barite was made in a 14 ppg fluid, inwhich the yield point of the fluid has been adjusted such that it is thesame between the two fluids before ageing.

PRODUCT Fluid A Fluid B PCC barite (2.4 sg) lbs/bbl 265 API baritelbs/bbl 265 Freshwater lbs/bbl 238 293 Soda Ash lbs/bbl 0.5 0.5 KOHlbs/bbl 0.5 0.5 PolyPlus RD lbs/bbl 0.5 0.5 PolyPac UL 2.0 2.0 Duovislbs/bbl 1.0 0.75 KCl lbs/bbl 8.0 8.0

The fluids were heat aged dynamically for 16 hrs at 150° F. Thefollowing table presented exemplary results.

FANN 35 Fluid A Fluid B Reading (120° F.) Initial Aged Initial Aged 600rpm 64 61 80 72 300 rpm 42 39 50 43 200 rpm 32 32 33 32 100 rpm 22 21 2421  6 rpm 6 5 6 6  3 rpm 4 4 4 4 PV (cps) 22 22 30 29 YP (lbs/100 sq.ft) 20 17 20 14 10 sec gel 5 5 5 5 (lbs/100 sq. ft) 10 min gel 17 11 6 6(lbs/100 sq. ft) API Fluid Loss 2.8 4.7 (ml/30 min) VST ppg 0.21 1.33

Upon review of the above, one of skill in the art should understand thatthe plastic viscosity for the polymer coated colloidal barite fluidswere lower and thus more desirable. The Viscometer Sag Test (VST) is analternative method for determining sag; in drilling fluids and isdescribed in American Society of Mechanical Engineers Magazine (1991) byD. Jefferson. As idicated above, the VST values for Fluid A, containingthe polymer coated colloidal barite of this invention is lower than thatof Fluid B formulated with untreated, API barite.

EXAMPLE 6

The long term thermal stability of the colloidal barite fluids of thepresent invention are shown in the following example at 17.34 ppg.ECF-614 additive is an organophilic clay additive available from M-IDrilling Fluids.

PRODUCT Fluid A PCC barite (2.4 sg) lbs/bbl 682 Freshwater lbs/bbl 53.5ECF-614 lbs/bbl 2.0

The fluid was heat aged statically for 4 days at 350° F. The followingtable provides exemplary results.

Fluid A FANN 35 Reading (120° F.) Initial Aged 600 rpm 107 45 300 rpm 6428  6 rpm 7 3  3 rpm 5 2 PV (cps) 43 17 YP (lbs/100 sq. ft) 21 11 10 secgel (lbs/100 sq. ft) 6 4 10 min gel (lbs/100 sq. ft) 10 11 Sag Factor0.503

Upon review of the above data one of skill in the art should understandand appreciate the long term thermal stability of the colloidal baritefluids of the present invention.

EXAMPLE 7

This test was carried out to show the feasibility of 24 ppg [2.87 g/cm³]slurries (0.577 Volume fraction). Each fluid contained the followingcomponents: fresh water 135.4 g, barite 861.0 g, IDSPERSE XT 18.0 g. Thebarite component was varied in composition according to the followingtable.

TABLE IV API grade Colloidal # barite (%) barite (%) 9 100 0 10 90 10 1180 20 12 75 25 13 60 40 14 0 100

TABLE V Viscosity at various shear rates (rpm of agitation): DialPlastic Yield Point reading or “Fann Units” for: Viscosity lb/100 ft² #600 300 200 117 100 59 30 6 3 mPa · s (Pascals) 9 *os 285 157 66 56 2610 3 2 10 245 109 67 35 16 13 7 3 2 136 −27 (−13) 11 171 78 50 28 23 107 3 2 93 −15 (−7)  12 115 55 36 19 17  8 5 3 2 60 −5 (−2) 13  98 49 3421 20 14 10 4 3 49 0 14 165 84 58 37 32 22 18 5 3 81   3 (−1.5) *os =off-scale

The results provided in table V show that API grade barite due to itsparticle size and the high volume fraction required to achieve high mudweights exhibit dilatancy i.e. high plastic and apparent viscosity andnegative yield values.

Introduction of fine grade materials tends to stabilize the flow regimekeeping it laminar at higher shear rates: plastic viscosity decreasesmarkedly and yield point changes from negative to positive. Nosignificant increase in low-shear rate viscosity (@3 rpm) is caused bythe colloidal barite.

These results show that the colloidal weight material of this inventionmay advantageously be used in conjunction with conventional API barite.

EXAMPLE 8

An eighteen pound per gallon [2.15 g/cm³] slurry of weighting agentaccording the present invention was formulated and subsequentlycontaminated with a range of common contaminants and hot rolled at 300°F. (148.90°C.). The rheological results of before (BHR) and after hotrolling (AHR) are presented below. The system shows excellent resistanceto contaminants, low controllable rheology and gives fluid loss controlunder a standard API mud test as shown in following table VI. Anequivalent set of fluids were prepared using API conventional baritewithout the polymer coating as a direct comparison of the two particletypes (Table VII).

TABLE VI (New barite) Viscosity (Fann Units) at various shear rates YPFluid (rpm of agitation: PV lb/100 ft² loss 600 300 200 100 6 3 mPa · s(Pascals) ml no contaminant BHR 21 11 8 4 1 1 10   1 (0.5) nocontaminant AHR 18 10 7 4 1 1 8 2 (1) 5.0 +80 ppb NaCl BHR 41 23 16 10 21 18   5 (2.5) +80 ppb NaCl AHR 26 14 10 6 1 1 12 2 (1) 16 +30 ppb OCMA¹BHR 38 22 15 9 2 1 16 6 (3) +30 ppb OCMA AHR 26 14 10 6 1 1 12 2 (1) 6.8 +5 ppb Lime BHR 15 7 5 3 1 1 8   −1 (−0.5)  +5 ppb Lime AHR 10 5 4 2 11 5 0 6.4 ¹OCMA = Ocma clay, a fine particle ball clay commonly used toreplicate drilled solids contamination acquired from shale sedimentsduring drilling

TABLE VII (Conventional API barite) Viscosity (Fann Units) at variousshear rates YP Fluid (rpm of agitation: PV lb/100 ft² loss 600 300 200100 6 3 mPa · s (Pascals) ml no contaminant BHR 22 10 6 3 1 1 12 −2 nocontaminant AHR 40 24 19 11 5 4 16 8 Total¹ +80 ppb NaCl BHR 27 13 10 62 1 14 −1 +80 ppb NaCl AHR 25 16 9 8 1 1 9 7 Total¹ +30 ppb OCMA BHR 6955 49 43 31 26 14 31 +30 ppb OCMA AHR 51 36 31 25 18 16 15 21 Total²  +5ppb Lime BHR 26 14 10 6 2 1 12 2  +5 ppb Lime AHR 26 14 10 6 1 1 12 2Total¹ ¹Total fluid loss within 30 seconds ²Total fluid loss within 5minutes.

A comparison of the two sets of data show that the weighting agentaccording the present invention (new barite) has considerable fluid losscontrol properties when compared to the API barite. The API barite alsoshows sensitivity to drilled solids contamination whereas the new baritesystem is more tolerant.

EXAMPLE 9

An experiment was conducted to demonstrate the ability of the newweighting agent to formulate drilling muds with densities above 20 poundper gallon [2.39 g/cm³].

Two twenty two pound per gallon [2.63 g/cm³] mud systems wereformulated, the weighting agents comprised a blend of 35% w/w new bariteweighting agent with 65% w/w API grade barite (Fluid #1) weighting agentand 100% API grade barite (fluid #2), both with 11.5 pound per barrel[32.8 kg/m³] STAPLEX 500 (mark of Schlumberger, shale stabilizer), 2pound per barrel [5.7 kg/m³] IDCAP (mark of Schlumberger, shaleinhibitor), and 3.5 pound per barrel [10 kg/m³] potassium chloride. Theother additives provide inhibition to the drilling fluid, but heredemonstrate the capacity of the new formulation to cope with anysubsequent polymer additions. The fluid was hot rolled to 200° F. (93.3°C.). Results are provided in table VIII.

TABLE VIII Viscosity (Fann Units) Yield at various shear rates PointFluid (rpm of agitation: PV lb/100 ft² loss 600 300 200 100 6 3 mPa · s(Pascals) ml Before Hot Rolling (#1) 110 58 46 30 9 8 52   6 (2.9) AfterHot Rolling (#1) 123 70 52 30 9 8 53  17 (8.1) 8.0 Before Hot Rolling(#2) 270 103 55 23 3 2 167 −64 (−32) After Hot rolling (#2) os 177 11047 7 5 12.0 os: off-scale

The 100% API grade barite has very high plastic viscosity and is in factturbulent as demonstrated by the negative yield point. After hot rollingthe rheology is so high that it is off scale.

EXAMPLE 10

This experiment demonstrates the ability of the new weighting agent ofthe present invention to lower the viscosity of fluids. The weightingagent is 100% colloidal barite according the present invention. Fluid#15 is based on synthetic oil (Ultidrill, Mark of Schlumberger, a linearalpha-olefin having 14 to 16 carbon atoms). Fluid #16 is a water-basedmud and includes a viscosifier (0.5 ppb IDVIS, Mark of Schlumberger, apure xanthan gum polymer) and a fluid loss control agent (6.6 ppb IDFLOMark of Schlumberger). Fluid #15 was hot rolled at 200° F. (93.3° C.),fluid #16 at 250° F. (121.1° C.). A results are shown table IX.

TABLE IX Viscosity (Fann Units) at various shear Gel¹ Yield Point rates(rpm of agitation: PV lbs/100 ft² lbs/100 ft² 600 300 200 100 6 3 mPa ·s (Pascals) (Pascals) #15:13.6 ppg 39 27 23 17 6 5 12 7/11 15 [1.63g/cm³] #16:14 ppg 53 36 27 17 6 5 17 5/— 19 [1.67 g/cm³] ¹A measure ofthe gelling and suspending characteristics of the fluid, determined at10 sec/10 min using a Fann viscosimeter.

Even though the formulation was not optimized, this test makes clearthat the new weighting agent provides a way to formulate brine analoguesfluids useful for slimhole applications or coiled tubing drillingfluids. The rheology profile is improved by the addition of colloidalparticles.

EXAMPLE 11

An experiment was conducted to demonstrate the ability of the newweighting agent to formulate completion fluids, were density control andhence sedimentation stability is a prime factor. The weighting agent iscomposed of the new colloidal barite according to the present inventionwith 50 pound per barrel [142.65 kg/m³] standard API grade calciumcarbonate, which acts as bridging solids. The 18.6 ppg [2.23 g/cm³]fluid was formulated with 2 pound per barrel [5.7 kg/m³] PTS 200 (markof Schlumberger, pH buffer) The static aging tests were carried out at400° F. (204.4° C.) for 72 hours. The results shown in the table below,before (BSA) and after (ASA) static aging reveal good stability tosedimentation and rheological profile.

Viscosity (Fann Units) at various shear rates YP (rpm of agitation:lb/100 ft² Free water* 600 300 200 100 6 3 PV mPa · s (Pascals) ml 18.6ppg 37 21 15 11 2 1 16 5 (2.5) — BSA 18.6 ppg 27 14 11 6 1 1 13 1 (0.5)6 ASA

EXAMPLE 12

This experiment demonstrates the ability of the new weighting agent toformulate low viscosity fluids and show it's tolerance to pH variations.The weighting agent is composed of the new colloidal barite according tothe present invention. The 16 ppg [1.91 g/cm³] fluid was formulated withcaustic soda to adjust the pH to the required level, with the subsequentfluid rheology and API filtration tested. The results shown in the tablebelow reveal good stability to pH variation and rheological profile.

Viscosity (Fann Units) at Yield various shear rates PV Point Fluid (rpmof agitation: mPa · lbs/100 ft² Loss PH 600 300 200 100 6 3 s (Pascals)ml 8.01 14 7 5 3 7 0(0)   8.4 9.03 14 8 5 3 6 2(1)   8.5 10.04 17 9 6 38 1(0.5) 7.9 10.97 17 9 6 3 8 1(0.5) 7.9 12.04 19 10 7 4 1 1 9 1(0.5)8.1

EXAMPLE 13

This experiment demonstrates the ability of the new weighting agent toformulate low rheology HTHP water base fluids. The weighting agent iscomposed of the new colloidal barite according to the present invention,with 10 pounds per barrel [28.53 kg/m³] CALOTEMP (mark of Schlumberger,fluid loss additive) and 1 pound per barrel [2.85 kg/m³] PTS 200 (markof Schlumberger, pH buffer). The 17 ppg [2.04 g/ kg/m³] and 18 ppg [2.16g/cm³] fluids were static aged for 72 hours at 250° F. (121° C.). Theresults shown in the table below reveal good stability to sedimentationand low rheological profile with the subsequent filtration tested.

Viscosity (Fann Units) Yield at various shear rates Point Free FluidDensity (rpm of agitation: PV lbs/100 ft² Water Loss ppg PH 600 300 200100 16 3 mPa · s (Pascals) ml ml 17 7.4 28 16 11 6 1 1 12 4 (2) 10 3.118 7.5 42 23 16 10 1 1 19 4 (2) 6 3.4

EXAMPLE 14

This experiment illustrates the ability of the fluids formulatedutilizing the polymer coated colloidal solid materials of the presentinvention to be pumped in a commercially available apparatus forinjecting viscous brine fluids into a casing annulus as part of a casingannulus pressure control program. The test apparatus was an unmodifiedCARS.TM. unit commercially available from ABB Vetco, having 500 feet ofhose on the reel, a small hose inner diameter of 0.2 inches, a hosefitting diameter of 0.1 inches, and a nylon ball of 0.25 inch diameter.A fluid in accordance with the present invention was formulated having adensity of 21.5 ppg and pumped through the test unit in accordance withall the proper procedures. The following table summarizes exemplarydata:

High Inlet Air Low Outlet Outlet Total Pressure Pressure Pressure FlowElapsed Calc Flow (PSI) (PSI) (PSI) (L) Time (min) (GPM) Comments 1301000 1700 1.3 4 0.12 Nylon ball in nozzle 130 1500 2000 1.7 2 0.32 Nylonball in nozzle 130 1000 3000 1.4 2 0.26 Nylon ball in nozzle 130 11002900 1.4 2 0.26 Nylon ball in nozzle 130 1100 2900 1.3 2 0.25 Nylon ballin nozzle w/VR Plug 130 700 2900 0.9 2 0.17 Nylon ball in nozzle w/VRPlug 130 1000 3000 1.3 2 0.25 VR Plug-No ball 130 800 2200 1 2 0.19 NoBall or VR Plug 110 1400 2400 8 1:32 1.97 Water, VR Plug, No Ball

A similar test was carried out using a larger hose having a 0.670 inchinner diameter, a hose fitting of 0.25 inches inner diameter; a nozzleof 0.67 VPN and spring #H30085-46. A fluid in accordance with thepresent invention was formulated having a density of 21.5 ppg and pumpedthrough the test unit in accordance with all the proper procedures. Thefollowing table summarizes exemplary data:

High Inlet Air Low Outlet Outlet Total Pressure Pressure Pressure FlowElapsed Calc Flow (PSI) (PSI) (PSI) (L) Time (min) (GPM) Comments 801200 1800 8 2:16 1.33 No nozzle or VR Plug 60 700 1200 8 2:38 1.15 Nonozzle or VR Plug 100 1750 2000 8 1:30 2.01 No nozzle or VR Plug 1101600 2000 8 1:23 2.18 No nozzle or VR Plug 120 1800 2300 8 1:40 1.81Nozzle, Spring, No VR Plug 110 1700 2000 8 1:33 1.95 Nozzle, Spring, NoVR Plug 110 1700 2000 8 1:34 1.93 Nozzle, Spring, No VR Plug 110 15002300 8 1:44 1.74 Nozzle, Spring, VR Plug 110 1500 2100 8 1:49 1.66Nozzle, Spring, VR Plug 110 1500 2200 8 1:53 1.6 Nozzle, Spring, VR Plug

One of ordinary skill in the art should understand and appreciate inview of the above data that fluids including the polymer dispersantcoated colloidal barite of the present invention can be readily pumpedand injected into the casing annulus using commercially availabletechnologies. It should also be appreciated that in contrast that if onewere to attempt a similar experiment with API barite or finely milledbarite, the particle sizes and the viscosity of either fluids wouldsubstantially prevent obtaining the above results.

EXAMPLE 15

This experiment illustrates the compatibility of a 22.4 ppg fluidformulated in accordance with the teachings of the present inventionwith a 17.6 ppg field lignosulfonate annulus fluid. The compatibilitytest consisted of measuring the rheology of the colloidal barite testfluid sample at 100, 120 and 150° F. and then measuring the rheology ofa 17.6 ppg lignosulfonate filed mud at 100, 120 and 150° F. The sampleswere then combined in the following ratios 75:25, 50:50 and 25:75(colloidal barite test fluid to lignosulfonate mud) and once again therheology was measured at the three temperatures listed. Exemplary datais provided below in the following tables:

Sample Colloidal barite solution (22.4 ppg) Rheology Temp. ° F. 100 120150 600 rpm 186 141 110 300 rpm 94 74 60 200 rpm 65 50 44 100 rpm 37 3127  3 rpm 6 6 6  6 rpm 5 5 5 plastic viscosity 92 67 50 (centipoise)Yield Point 2 7 10 (100 ft²) Gels, 10 Sec. 5 5 5 Gels, 10 min. 11 11 12Sample 17.6 Field Lignosulfonate Annulus Fluid Rheology Temp ° F. 100120 150 600 rpm 103 90 76 300 rpm 57 51 45 200 rpm 42 37 52 100 rpm 2524 20  3 rpm 4 4 4  6 rpm 3 3 3 plastic viscosity 46 39 31 (centipoise)Yield Point 11 12 14 (100 ft²) Gels, 10 Sec. 5 5 5 Gels, 10 min. 12 1417 22.4 ppg Colloidal barite:17.6 ppg Field Sample Lignosulfonate 75:25Rheology Temp. ° F. 100 120 150 600 rpm 200 174 147 300 rpm 116 99 87200 rpm 86 74 66 100 rpm 53 46 42  3 rpm 14 12 12  6 rpm 11 10 10plastic viscosity 84 75 60 (centipoise) Yield Point 32 24 27 (100 ft²)Gels, 10 Sec. 17 15 14 Gels, 10 min. 57 57 60 22.4 ppg Colloidalbarite:17.6 ppg Field Sample Lignosulfonate 50:50 Rheology Temp. ° F.100 120 150 600 rpm 179 154 135 300 rpm 102 90 82 200 rpm 75 69 63 100rpm 45 43 41  3 rpm 10 11 12  6 rpm 8 9 10 plastic viscosity 77 64 53(centipoise) Yield Point 25 26 29 (100 ft²) Gels, 10 Sec. 13 12 16 Gels,10 min. 50 52 58 22.4 ppg Colloidal barite:17.6 ppg Field SampleLignosulfonate 25:75 Rheology Temp. ° F. 100 120 150 600 rpm 124 110 91300 rpm 72 65 55 200 rpm 54 49 42 100 rpm 33 31 28  3 rpm 6 7 6  6 rpm 56 5 plastic viscosity 52 45 36 (centipoise) Yield Point 20 20 19 (100ft²) Gels, 10 Sec. 7 10 11 Gels, 10 min. 26 31 32

Upon review and careful examination of the above data, one of skill inthe art should understand and appreciate that the compatibility of thefluids at a test temperature of 100° F. It should also be noted that thePV decreases from the colloidal barite solution standard when mixed withthe field sample. It was noted that the YP increase to a maximum of 32100 ft/lbs.sup.2, however, a skilled person would understand that iswell within what would be considered pumpable. An increase in the gelstrengths should also be noted, however, this also is within anacceptable range. Upon considering the entirety of the above data, oneof skill in the art should be able to understand and appreciate thecompatibility of the colloidal barite fluids of the present inventionand lignosulfonate annulus fluids.

EXAMPLE 16

This experiment illustrates the ability of the fluids of the presentinvention to displace a 17.6 ppg field lignosulfonate annulus fluid.This test consisted of placing 50 mis of the 17.6 ppg lignosulfonatefield mud in a 100 ml graduated cylinder. The 22.4 ppg colloidal baritefluid of the present invention was loaded into a 60 ml syringe with a 6″long blunt nose needle. The tip of the needle was placed inside thegraduated cylinder to 5 ml below the 50 ml mark and the colloidal baritefluid was then injected into the field mud sample at a rate of about 50ml/minute. The sample was then allowed to stand at room temperature forabout 5 minutes. After the time had expired a hollow glass barrel wascarefully inserted into the sample and run to bottom. The hollow glassbarrel was then capped and pulled from the graduate cylinder in a mannerto obtain a sample of the fluids in the graduated cylinder. By visualobservation, one of skill in the art should notice that the bottom halfof the hollow glass cylinder contains the colloidal barite fluid of thepresent invention. This can be determined visually by the color changefrom the colloidal barite fluid (light tan to white) to the field mud(very dark brown).

In order to quantify these findings the test was rerun but this timeinstead of running in the hollow glass cylinder, a 20 ml syringe withthe long blunt nose needle was run in the sample. The needle was run tothe bottom of the graduate cylinder were a 25 ml sample was extractedfrom the bottom of the graduated cylinder. This sample was then placedinto a 20 ml picnometer and weighed on a Mettler bench top scale. Thespecific gravity of the sample was determined to be 2.694 which whenconverted is a density of 22.47 ppg. The original sample weight was 22.5ppg

One of skill in the art should understand and appreciate that this testdemonstrates the ability of the colloidal barite solutions of thepresent invention to not only fall through the existing 17.6 ppglignosulfonate filed sample quickly but also to remain intact and not bedispersed as it is falling through the field mud. The significance ofthis result should be appreciated by such a skilled artisan as anindication that little if any contamination and/or reduction in thedensity value of the colloidal barite fluids of the present inventionoccurs as a result of mixing. For this reason, one of skill in the artshould understand and appreciate that injection of the colloidal baritefluids of the present invention into a casing annular space shouldresult in minimal dilution of the colloidal barite fluid and the bottomup displacement of any fluids existing in the casing annulus.

While not intending to be bound by any specific theory of action, it isbelieved that the formation of the colloidal solid material by a highenergy wet process, in which API Barite of median particle size of 25-30micron is reduced to a median particle size of less than 2 microns, ismore efficient when the milling is done at high density, normallygreater than 2.1 sg, preferably at 2.5 sg. At these high densities, thevolume or mass fraction of barite is very high. For example, at aspecific gravity of 2.5, 100 kg of the final product contains about 78kg barite. However, the resulting slurry still remains fluid. Thepresence of the surface active polymer during the course of thecomminution process is an important factor in achieving the results ofthe present invention. Further, the surface active polymer is designedto adsorb onto surface sites of the barite particles. In the grinder,where there is a very high mass fraction of barite, the polymer easilyfinds it way onto the newly formed particle surfaces. Once the polymer‘finds’ the barite—and in the environment of the grinder it has everychance to do so—a combination of the extremely high energy environmentin the wet grinding mill (which can reach 85 to 90° C. inside the mill),effectively ensures that the polymer is ‘wrapped’ around the colloidalsize barite. As a result of this process it is speculated that nopolymer ‘loops’ or ‘tails’ are hanging off the barite to get attached,snagged, or tangled up with adjacent particles. Thus it is speculatedthat the high energy and shear of the grinding process ensures thepolymer remains on the barite permanently and thus the polymer doesn'tdesorb, or become detached.

This theory of action is supported by the observation that adding thesame polymer to the same mass fraction of colloidal barite at roomtemperature and mixing with the usual lab equipment provides verydifferent results. Under such conditions it is believed that polymerdoesn't attach itself to the surface properly. This may be due topresence of a sphere of hydration or other molecules occupying thesurface binding sites. As a result the polymeric dispersant is notpermanently ‘annealed’ to the surface, and thus, the rheology of thesuspension is much higher. It has also been observed that the suspensionis not so resistant to other contaminants possibly because the polymerwants to detach itself from the barite and onto these more reactivesites instead.

In view of the above disclosure, one of ordinary skill in the art shouldunderstand and appreciate that one illustrative embodiment of thepresent invention includes a method of controlling the pressure of acasing annulus in a subterranean well. In one such illustrative method,the method includes, injecting into the casing annulus a compositionincluding a base fluid, and a polymer coated colloidal solid material.The polymer coated colloidal solid material includes: a solid particlehaving an weight average particle diameter (d₅₀) of less than twomicrons, and a polymeric dispersing agent absorbed to the surface of thesolid particle during the course of the cominution process. The basefluid utilized in the above illustrative embodiment can be an aqueousfluid or an oleaginous fluid and preferably is selected from: water,brine, diesel oil, mineral oil, white oil, n-alkanes, synthetic oils,saturated and unsaturated poly(alpha-olefins), esters of fatty acidcarboxylic acids and combinations and mixtures of these and similarfluids that should be apparent to one of skill in the art. Suitable andillustrative colloidal solids are selected such that the solid particlesare composed of a material of specific gravity of at least 2.68 andpreferably are selected from barium sulfate (barite), calcium carbonate,dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate,combinations and mixtures of these and other suitable materials thatshould be well known to one of skill in the art. In one preferred andillustrative embodiment, the polymer coated colloidal solid material hasa weight average particle diameter (d₅₀) less than 2.0 microns. Anotherillustrative embodiment contains at least 60% of the solid particleshave a diameter less than 2 microns or alternatively more than 25% ofthe solid particles have a diameter less than 2 microns. The polymericdispersing agent utilized in one illustrative and preferred embodimentis a polymer of molecular weight of at least 2,000 Daltons. In anothermore preferred and illustrative embodiment, the polymeric dispersingagent is a water soluble polymer is a homopolymer or copolymer ofmonomers selected from the group comprising: acrylic acid, itaconicacid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonicacid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonicacid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate,and wherein the acid monomers may also be neutralized to a salt.

Another illustrative embodiment of the present invention includes amethod of controlling the pressure of a casing annulus in a subterraneanwell, the method including inserting into the casing annulus asufficient amount of a flexible tubing so as to reach a predetermineddepth, and pumping into the flexible tubing a pressure controlcomposition so as to inject an effective amount of the pressure controlcomposition into the casing annulus so as to substantially displace anyexisting fluid in the casing annulus. In such an illustrativeembodiment, the pressure control composition includes: a base fluid, anda polymer coated colloidal solid material, in which the polymer coatedcolloidal solid material includes: a solid particle having an weightaverage particle diameter (d₅₀) of less than two microns, and apolymeric dispersing agent absorbed to the surface of the solidparticle. Another illustrative embodiment contains at least 60% of thesolid particles have a diameter less than 2 microns or alternativelymore than 25% of the solid particles have a diameter less than 2microns. One preferred and illustrative embodiment includes a base fluidthat is an aqueous fluid or an oleaginous fluid and which is preferablyselected from, water, brine, diesel oil, mineral oil, white oil,n-alkanes, synthetic oils, saturated and unsaturatedpoly(alpha-olefins), esters of fatty acid carboxylic acids andcombinations and mixtures of these and other similar fluids that shouldbe apparent to one of skill in the art. A preferred and illustrativeembodiment includes solid particles composed of a material having aspecific gravity of at least 2.68 and more preferably the colloidalsolid is selected from barium sulfate (barite), calcium carbonate,dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate andcombinations and mixtures of these and other similar solids that shouldbe apparent to one of skill in the art. The polymeric dispersing agentutilized in the present illustrative embodiment is preferably a polymerof molecular weight of at least 2,000 Daltons. Alternatively, thepolymeric dispersing agent is a water soluble polymer is a homopolymeror copolymer of monomers selected from the group comprising: acrylicacid, itaconic acid, maleic acid or anhydride, hydroxypropyl acrylatevinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide,styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether andvinyl acetate, and wherein the acid monomers may also be neutralized toa salt.

In addition to the above illustrative methods, the present invention isalso directed to a composition that includes a base fluid and a polymercoated colloidal solid material. The polymer coated colloidal solidmaterial is formulated so as to include a solid particle having anweight average particle diameter (d₅₀) of less than two microns; and apolymeric dispersing agent coated onto the surface of the solidparticle. One illustrative embodiment includes a base fluid that iseither an aqueous fluid or an oleaginous fluid and preferably isselected from, water, brine, diesel oil, mineral oil, white oil,n-alkanes, synthetic oils, saturated and unsaturatedpoly(alpha-olefins), esters of fatty acid carboxylic acids, combinationsand mixtures of these and other similar fluids that should be apparentto one of skill in the art. It is preferred in one illustrativeembodiment that the solid particles are composed of a material ofspecific gravity of at least 2.68 and more preferably that the colloidalsolid is selected from barium sulfate (barite), calcium carbonate,dolomite, ilmenite, hematite, olivine, siderite, strontium sulfate andcombinations and mixtures of these and other similar solids that shouldbe apparent to one of skill in the art. The polymer coated colloidalsolid material utilized in one illustrative and preferred embodiment hasa weight average particle diameter (d₅₀) less than 2.0 microns. Anotherillustrative embodiment contains at least 60% of the solid particleshave a diameter less than 2 microns or alternatively more than 25% ofthe solid particles have a diameter less than 2 microns. A polymericdispersing agent is utilized in a preferred and illustrative embodiment,and is selected such that the polymer preferably has a molecular weightof at least 2,000 Daltons. Alternatively the illustrative polymericdispersing agent may be a water soluble polymer is a homopolymer orcopolymer of monomers selected from the group comprising: acrylic acid,itaconic acid, maleic acid or anhydride, hydroxypropyl acrylatevinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide,styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether andvinyl acetate, and wherein the acid monomers may also be neutralized toa salt.

One of skill in the art should understand and appreciate that thepresent invention further includes a method of making the polymer coatedcolloidal solid material described above. Such an illustrative methodincludes grinding a solid particulate material and a polymericdispersing agent for a sufficient time to achieve an weight averageparticle diameter (d₅₀) of less than two microns; and so that thepolymeric dispersing agent is absorbed to the surface of the solidparticle. Preferably the illustrative grinding process is carried out inthe presence of a base fluid. The base fluid utilized in oneillustrative embodiment is either an aqueous fluid or an oleaginousfluid and preferably is selected from, water, brine, diesel oil, mineraloil, white oil, n-alkanes, synthetic oils, saturated and unsaturatedpoly(alpha-olefins), esters of fatty acid carboxylic acids andcombinations thereof. In one illustrative embodiment the solidparticulate material is selected from materials having of specificgravity of at least 2.68 and preferably the solid particulate materialis selected from barium sulfate (barite), calcium carbonate, dolomite,ilmenite, hematite, olivine, siderite, strontium sulfate, combinationsand mixtures of these and other similar solids that should be apparentto one of skill in the art. The method of the present invention involvesthe grinding the solid in the presence of a polymeric dispersing agent.Preferably this polymeric dispersing agent is a polymer of molecularweight of at least 2,000 Daltons. The polymeric dispersing agent in onepreferred and illustrative agent is a water soluble polymer that is ahomopolymer or copolymer of monomers selected from the group comprising:acrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropylacrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid,acrylamide, styrene sulphonic acid, acrylic phosphate esters, methylvinyl ether and vinyl acetate, and wherein the acid monomers may also beneutralised to a salt.

It should also be appreciated by one of skill in the art that theproduct of the above illustrative process is considered part of thepresent invention. As such one such preferred embodiment includes theproduct of the above illustrative process in which the polymer coatedcolloidal solid material has a weight average particle diameter (d₅₀)less than 2.0 microns. Another illustrative embodiment contains at least60% of the solid particles have a diameter less than 2 microns oralternatively more than 25% of the solid particles have a diameter lessthan 2 microns.

While the apparatus, compositions and methods of this invention havebeen described in terms of preferred or illustrative embodiments, itwill be apparent to those of skill in the art that variations may beapplied to the process described herein without departing from theconcept and scope of the invention. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the scope and concept of the invention as it is set out in thefollowing claims.

1. A method of formulating a wellbore fluid comprising: preparing apolymer coated colloidal solid material, wherein the preparation stepcomprises grinding a solid particulate material and a polymericdispersing agent for a sufficient time to achieve a weight averageparticle diameter d50 of less than 2 microns but not more than 5% of theparticles are less than 0.1 micron in diameter, wherein the polymericdispersing agent has a molecular weight greater than 10,000; andsuspending the polymer coated wellbore additive colloidal solid materialin the wellbore fluid.
 2. The method of claim 1, wherein the grinding iscarried out in the presence of an oleaginous base fluid.
 3. The methodof claim 2, wherein the oleaginous base fluid is selected from the groupconsisting of diesel oil, mineral oil, white oil, n-alkanes, syntheticoils, saturated and unsaturated poly (alpha-olefins), ester oils, andcombinations thereof.
 4. The method of claim 1 wherein the polymericdispersing agent coats the surface of the solid particles.
 5. The methodof claim 1, wherein the solid particulate material is selected from thegroup consisting of barium sulfate, calcium carbonate, dolomite,ilmenite, hematite, olivine, siderite, strontium sulfate andcombinations thereof.
 6. The method of claim 1, wherein the solidparticulate material is selected from materials having of specificgravity of at least 2.68.
 7. The method of claim 1, wherein thepolymeric dispersing agent is a polymer of molecular weight less thanabout 200,000.
 8. The method of claim 1, wherein the polymericdispersing agent is a water soluble polymer that comprises monomersselected from the group consisting of: acrylic acid, itaconic acid,maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid,acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonic acid,acrylic phosphate esters, methyl vinyl ether, vinyl acetate, and saltsthereof.
 9. The product of the process of claim
 1. 10. The method ofclaim 1, wherein the grinding is carried out in the presence of anaqueous base fluid.
 11. The method of claim 1, wherein the polymericdispersing agent is selected from the group consisting of phospholipids,synthetic polymers, carboxylic acids of molecular weight of at least150, alkylbenzene suphonic acids, alkane sulphonic acids, linearalpha-olefin sulphonic acid, and the alkaline earth metal salts of theacids.
 12. A method of making a wellbore fluid, the method comprising:grinding a solid particulate material and a polymeric dispersing agentfor a sufficient time to provide a portion of the resulting polymercoated colloidal solid material having a particle diameter less than 2.0microns, wherein the polymeric dispersing agent has a molecular weightgreater than 10,000; and suspending the polymer coated wellbore additivecolloidal solid material in the wellbore fluid.
 13. The method of claim12, wherein the grinding is carried out in the presence of an oleaginousbase fluid.
 14. The method of claim 13, wherein the oleaginous basefluid is selected from the group consisting of diesel oil, mineral oils,white oils, n-alkanes, synthetic oils, saturated and unsaturatedpoly(alpha-olefins), esters oils, and combinations thereof.
 15. Theproduct of the process of claim 10, wherein greater than 25% of thepolymer coated colloidal solid material has a particle diameter lessthan 2 microns.
 16. The product of the process of claim 10, wherein atleast 60% of the polymer coated colloidal solid material has a particlediameter less than 2 microns.
 17. The method of claim 10, wherein thegrinding is carried out in the presence of an aqueous base fluid. 18.The method of claim 10, wherein the polymeric dispersing agent isselected from the group consisting of phospholipids, synthetic polymers,carboxylic acids of molecular weight of at least 150, alkylbenzenesuphonic acids, alkane sulphonic acids, linear alpha-olefin sulphonicacid, and the alkaline earth metal salts of the acids.
 19. The method ofclaim 12, wherein the polymeric dispersing agent is a water solublepolymer that comprises monomers selected from the group consisting ofacrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropylacrylate vinylsulphonic acid, acrylamido 2- propane sulphonic acid,acrylamide, styrene sulphonic acid, acrylic phosphate esters, methylvinyl ether, vinyl acetate, and salts thereof.
 20. A method of making aa wellbore fluid, the method comprising: grinding a solid particulatematerial and a polymeric dispersing agent, the polymeric dispersingagent having a molecular weight greater than 10,000, for a sufficienttime such that the polymeric dispersing agent coats the surface of theresulting colloidal solid particles and less than 10% of the resultingcolloidal solid particles have a diameter greater than 10 microns; andincreasing the density the wellbore fluid with the resulting colloidalsolid particles.
 21. The method of claim 20, wherein the polymericdispersing agent is a polymer having a molecular weight in a range fromabout 40,000 to about 120,000.
 22. The product of the process of claim21.
 23. The method of claim 20, wherein the grinding is carried out inthe presence of an oleaginous base fluid.
 24. The method of claim 23,wherein the base fluid is selected from the group consisting of: dieseloil, mineral oil, white oil, n-alkanes, synthetic oils, saturated andunsaturated poly(alpha-olefins), ester oils and combinations thereof.25. The method of claim 20, wherein the colloidal solid material has aweight average particle diameter d50 less than 1.5 microns.
 26. Themethod of claim 1, wherein the solid particulate material is selectedfrom materials having of specific gravity of at least 2.68.
 27. Theproduct of the process of claim
 20. 28. The method of claim 20, whereinthe grinding is carried out in the presence of an aqueous base fluid.29. The method of claim 20, wherein the polymeric dispersing agent isselected from the group consisting of phospholipids, synthetic polymers,carboxylic acids of molecular weight of at least 150, alkylbenzenesuphonic acids, alkane sulphonic acids, linear alpha-olefin sulphonicacid, and the alkaline earth metal salts of the acids.
 30. The method ofclaim 20, wherein the polymeric dispersing agent is a water solublepolymer that comprises monomers selected from the group consisting ofacrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropylacrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid,acrylamide, styrene sulphonic acid, acrylic phosphate esters, methylvinyl ether, vinyl acetate, and salts thereof.